Touch-sensitive devices allow a user to conveniently interface with electronic systems and displays by reducing or eliminating the need for mechanical buttons, keypads, keyboards, and pointing devices. For example, a user can carry out a complicated sequence of instructions by simply touching an on-display touch screen at a location identified by an icon.
There are several types of technologies for implementing a touch-sensitive device including, for example, resistive, infrared, capacitive, surface acoustic wave, electromagnetic, near field imaging, etc. Capacitive touch-sensing devices have been found to work well in a number of applications. In many touch-sensitive devices, the input is sensed when a conductive object in the sensor is capacitively coupled to a conductive touch implement such as a user's finger. Generally, whenever two electrically conductive members come into proximity with one another without actually touching, a capacitance is formed therebetween. In the case of a capacitive touch-sensitive device, as an object such as a finger approaches the touch sensing surface, a tiny capacitance forms between the object and the sensing points in close proximity to the object. By detecting changes in capacitance at each of the sensing points and noting the position of the sensing points, the sensing circuit can recognize multiple objects and determine the characteristics of the object as it moves across the touch surface.
There are two known techniques used to capacitively measure touch. The first is to measure capacitance-to-ground, whereby a signal is applied to an electrode. The first method is referred to as self-capacitance herein. A touch in proximity to the electrode causes signal current to flow from the electrode, through an object such as a finger, to electrical ground.
According to various embodiments, a technique is used to capacitively measure touch is through mutual capacitance. Mutual capacitance touch screens apply a signal to a driven electrode, which is capacitively coupled to a receiver electrode by an electric field. Signal coupling between the two electrodes is reduced by an object in proximity, which reduces the capacitive coupling.